Today, the clean room, the clean bench, and the like are widely used for manufacturing a large scale integrated (LSI) circuit, a liquid crystal display (LCD) panel and so forth, all of which have to be carefully protected from specific impurities or contaminants contained in the ordinary air. In case of manufacturing products like these items, however, it is not rare that they are forced to be kept inside a carrier or a stocker in the form of a half-finished product after having passed through predetermined manufacturing steps. Consequently, they come to exposed to the inside atmosphere of the carrier or stocker until they are transferred to the next manufacturing step. Furthermore, it is also not rare that this intermediate storage or waiting state is continued over a period of several to several tens hours. Consequently, should the atmosphere of the clean room or the like be insufficiently cleaned, gaseous impurities contained therein would be deposited on the semiconductor substrate surface of LSI or glass substrate surface of LCD. Acid substances, alkaline substances, organic substances, and various dopants are considered as impurities that would give an ill influence to the performance of LSI or LCD when they are deposited on the substrate surface thereof. Especially, sulfur oxides SOx among acid substances reacts with ammonia gas contained, if any, in the clean room atmosphere to unfavorably form a thin cloudy matter consisting of ammonium sulfate over the surface of a silicon wafer, a glass substrate, a lens, a mirror, a hard disk, a magnetic disk, and so forth. Thus, this would give the ill influence to the manufacture of semiconductor elements, LCD substrates, and magnetic memory parts. The most popular sulfurous acid gas representing sulfur oxides may be generated from various sources. For instance, it may be generated from sulfuric acid that is used in the rinsing process carried out inside the clean room, and also it might be contained as an atmospheric pollutant or volcanic gas in the outdoor air introduced to the clean room. Boron B also may act as a contaminant against thin film transistors (TFT), and it contaminates the channel region of TFT to make the characteristic thereof worse.
According to one example of the actual measurement of various contaminants contained in the atmosphere of the ordinary clean room that is not provided with any chemical protective measure against gaseous contaminants, it is reported that there are contained in such atmosphere acid substances of about 100 ppt-1,000 ppt, basic substances of about 1,000 ppt-10,000 ppt, organic substances of about 1,000 ppt-10,000 ppt, and dopants of about 10 ppt-100 ppt, respectively.
According to the allowable concentration (ppt) of the chemical contaminant required for 0.25.mu. process (after '98), which is disclosed in the article entitled "Forecast Of Airborne Molecular Contamination Limits For The 0.25 Micron High Performance Logic Process" of Technology Transfer #95052812A-TR published by SEMATECH (U.S.A.) on May 31, 1995, severe limitations are imposed on the concentration of various chemical contaminants. That is, with regard to acid substances, they should be limited to less than 180 ppt in the salicidation process, and less than 5 ppt in the contact formation process. As for basic substances, they should be limited to less than 1 ppb in the photolithographic process. With respect to the dopant, it should be limited to less than 0.1 ppt for the pre-gate oxidation process. As to organic substances, they should be limited to less than 1 ppb in the pre-gate oxidation process, and less than 2 ppb in the contact formation process.
Among these four chemical contaminants, almost all of acid substances, basic substances and dopants are water-soluble, so that they can be removed by means of the wet type rinsing (scrubber) or by means of chemical adsorption using a chemical filter that is formed of ion-exchange fiber or activated charcoal, to the surface of which a selected chemical is impregnated (referred to as "chemical impregnated activated charcoal" hereinafter). A cylindrical case filled up with chemical impregnated activated charcoal is well known as the most simple configuration of a chemical filter using chemical impregnated activated charcoal. Activated charcoal impregnated with potassium permanganate solution has been used as the filter media of a chemical filter for removing sulfur oxides SOx. Chemical filters of other types are available. For instance, some chemical filters are made in the form of a felt or a nonwoven fabric that is formed by interweaving chemical impregnated fibrous activated charcoal with the polyester fiber, using an organic binder, and the others are formed in the shape of a block or a sheet by binding chemical impregnated activated charcoal powder to an air permeable urethane foam or a nonwoven fabric with a proper adhesive. The chemical filter using chemical impregnated activated charcoal is disclosed in the Japanese unexamined patent publication No. 61-103518, which is incorporated by reference herein. This filter uses the acid cation exchange fiber and the basic anion exchange fiber as its basic filter media, and is made in the form of a nonwoven fabric, a sheet, or a felt. Various acid and basic ions can be ion-exchanged and removed with this filter.
On one hand, organic substances are generally insoluble in water, so that they may be basically removed by means of the physical adsorption function of activated charcoal.
In case of the wet type air rinsing system, an atomizer will require a rather larger initial cost for installation thereof and also require a considerable running cost for generating a high pressure spray, which can not be ignored. Also, it should be noted that this system relates to the humidity and temperature control of the clean room air. Consequently, in case of circulating the air through the clean room, if it is rinsed by this system, it is additionally required to control the humidity and temperature of the circulating clean room air and to remove tiny water droplets contained therein immediately after being rinsed thereby. Furthermore, the rinsing water has to be always kept clean and free from any of bacteria, water soluble contaminants, and so forth, so that it is required, for instance, to additionally arrange other facilities for preventing bacteria from being generated and/or for condensing and separating water soluble contaminants.
In view of disaster prevention standpoint, however, it is not preferable, should rather be avoided, to install the chemical filter that uses chemical impregnated activated charcoal or ion exchange fiber, on the ceiling portion of the clean room, because they are inflammable. The prior art chemical filter generally includes, as its components, a nonwoven fabric, an organic binder for fixing activated charcoal to a sheet of filter media, a sealing member, and so forth. However, these components are apt to desorb gaseous organic impurities, which are readily mixed with the air once cleaned by passing through the chemical filter, and would never fail to give a wrong influence to the manufacture of semiconductor devices. Dusts flying away from the filter media are also one of wrong factors causing the product contamination. If a plurality of processes which might cause various gaseous inorganic impurities, are executed in one clean room, the chemical filter is naturally required to have an excellent filtering performance against such gaseous inorganic impurities. At the same time, it is strictly required that the chemical filter never desorbs any of gaseous organic impurities by itself. In general, in order to block dusts which might fly away from the chemical filter, a particulate air filter is installed on the downstream side of the chemical filter. However, if such particulate air filter desorbs some gaseous organic impurities by itself, the function of the chemical filter would be in vain even though it is so excellent. Furthermore, the prior art chemical filter including basic chemicals and metals fails to remove sulfurous acid gas sufficiently.
In case of the chemical filter using the ion exchange fiber, gaseous organic substances might be desorbed from various additive agents included in the polymer fiber forming the ion exchange fiber. In this case, it sometimes takes place that a part of the ion exchange group is desorbed as sulfonic acid gas, carboxylic acid gas, phosphoric acid gas, ammonia gas, or amine gas.
Furthermore, in case of filtering the clean room air including both of acid and basic inorganic impurities by means of the prior art chemical filter that uses chemical impregnated activated charcoal or ion exchange fiber, it is required to separately prepare both of an adsorptive media for removing acid inorganic impurities and an adsorptive media for removing basic inorganic impurities.
Accordingly, an object of the invention is to provide an air filter that is excellent from the standpoint of disaster prevention, and is capable of removing both of organic and inorganic impurities contained in the object air, and desorbs neither gaseous inorganic impurities nor gaseous organic impurities by itself. Furthermore, the other object of the invention is to provide a method for manufacturing such filters and a high efficiency clean room or the like installed with such filters as manufactured according to the above method.
Another object of the invention is to provide an air filter that is able to work for a long time and efficiently remove sulfurous acid gas which could not be sufficiently removed by the prior art chemical filter.